Sunday, 22 January 2012

Weasels, Weasels, Everywhere

Long-tailed weasel

The most successful body plan within the weasel family has been that for which it is best known: the small, slender, agile carnivore that feeds on prey too small for larger predators to bother with. In Europe and Canada, the best known such animals are probably common weasels and stoats, both remarkably widespread animals across the temperate regions of the northern hemisphere. But, of course, they are far from alone, just two among a total of fourteen currently recognised species of what, for lack of a better term, we can call "true weasels".

In North America, common (or "least") weasels and stoats are found in Canada and in the northwestern and northeastern corners of the US; they generally do not venture into warmer climes, as they do in many parts of Europe and Asia. That's because, in America, there is another species of weasel better adapted to those regions. This is the long-tailed weasel (Mustela frenata), and it, not the common weasel, is the one best known in most parts of the USA.
Like it's more northerly kin, it is a highly adaptable animal, and while it may prefer relatively open, brushy areas, it is happy to live anywhere from forests to farmland. As a result, while it avoids the harshest deserts, it is found in every one of the 48 contiguous states, as well as in the Great Plains and the southwestern mountains of Canada. Not only that, but is also inhabits much of Mexico, the whole of Central America, and an arc of territory stretching from Venezuela to Bolivia. So, this is an animal that is perfectly happy in jungles, mountains, plains, or pine forests - just about anywhere it can find rodents to eat and at least a passable supply of water.

Long-tailed weasels look much like stoats; the males are about the same size, although the females are noticeably smaller. They have a longer tail, of course, and somewhat yellowish (rather than white) underparts, and, in some parts of their range, they also have black markings on the face. They resemble stoats in many other respects, too. Where it's cold enough to be worthwhile, for instance, they moult in winter, producing a pure white coat, save for a black tip to the tail, which presumably serves to distract predators away from the more vital parts of their anatomy. Also like stoats, they breed in mid-summer, then hold the embryo in suspended animation until the following spring, so that as much food as possible is available when the mother finally gives birth.

Their small, slender body gives them a high metabolism that keeps them frenetically active, so that, while mainly nocturnal, they are frequently obliged to feed during the day because they burn up so many calories just staying alive. They feed mainly on rodents, spending much of their lives peeking into any hole or crevice they can squeeze into (and, given their size and shape, that's quite a few) to see if there's anything edible inside. They can, however, also take down larger prey, including rabbits, and, if there's a knothole in a hen house, domestic chickens. There does not, however, appear to be any truth in the myth that they drink the blood of their prey, except insofar as that's difficult to avoid while eating.

The white winter coat of northern long-tailed weasels is no thicker than their summer one; evidently, their small size and slender shape means that they lose heat so rapidly that this would make little difference, and instead, it's purely for camouflage against the snow. As a result, it has been suggested that watching how the geographic boundary between moulting weasels and those that stay brown all year moves, may, as the world continues to warm, give us some insights into short-term evolution. On the other hand, the pure white coat does make the animal valuable to fur trappers, and, indeed, much of the fur sold as "ermine" is actually from these creatures, and not from stoats.

Rather less is known of the other eleven species of "true" weasel, probably because none of them live in Europe or North America. They range in size from almost as small as the common weasel to nearly as large as a polecat, but they all have the same general body plan of a long, slender, torso and short legs. Most have the typical "weasel" coat pattern of a brown body with distinct, pale underparts, but there are exceptions. Although some of them do get paler in winter, none turn to pure white.

Apart from Australia and Antarctica, the only continent in which they have had little success is Africa. The only species that lives there is the Egyptian weasel (Mustela subpalmata), and that lives only in the irrigated farmlands of the lower Nile valley, in the extreme northeastern corner of the continent. We know almost nothing about it, other than that is so similar to the common weasel that it was long thought to be the same animal.

Siberian weasel

Most of the remaining species live in eastern Asia. The best studied are probably the Siberian weasels (Mustela sibirica), which are often hunted for their fur. Despite their name, they are also found in western Russia, and they are common throughout China and Korea, and can be found as far south as Laos. Sometimes called "kolinskis", they live in high altitude forests, and they are somewhat between other weasels and polecats in appearance, size, and habits, something that is born out by their apparent position on the evolutionary tree of the weasel family. Japanese weasels (Mustela itatsi) are extremely similar, and are probably close relatives; they live only on the Japanese islands.

Altai Mountain weasels (Mustela altaica) are, as their name suggests, similarly high altitude, but they prefer open alpine meadows, and, at best, only venture into the fringes of sparse woodland areas. They are found across, not just the Altai Mountains of western Mongolia, but also across other nearby mountain ranges, including the Himalayas, where they feed on mountain pikas and rodents.

Altai mountain weasel

Heading further south, we come to another mountain species, the yellow-bellied weasel (Mustela kathiah) of south China, northern India, and neighbouring regions south to Vietnam. They are apparently quite adaptable, being found from high pine forests to chilly, desolate wastelands, and even taking trips into nearby lowland areas, all of which means that they seem to be holding up quite well, despite being regularly hunted. Back-striped weasels (Mustela strigidorsa), which have a distinctive streak of pale fur down the middle of their back, live in much the same part of the world, but in jungle-covered hills, rather than in mountainous areas. Although back-striped weasels used to be thought extremely rare, and likely endangered, it seems that they're really just good at hiding - as well they might be, in the jungle - and, so far at least, there seems no reason to suppose they're at any risk, although their body parts are sometimes used in traditional medicine.

Still in the jungles of southeast Asia, we next come to the Malaysian weasels (Mustela nudipes), living in the Malaysian peninsula, and the islands of Borneo and Sumatra. They have a particularly distinctive appearance, being about the size of a ferret, with bright reddish-brown fur over most of the body, that contrasts with a pale, or even white, face. We know very little about them, but we know even less about their close neighbours, the Indonesian mountain weasels (Mustela lutreolina), similarly large animals that live somewhere among the isolated mountain peaks of Sumatra and Java and bear a remarkable resemblance to mink - only nine individuals have ever been seen by scientists.

Back-striped weasel

All of this indicates that, while more familiar species of weasel inhabit relatively cold environments, there are many that are perfectly at home in the depths of sweltering tropical jungles. While none have penetrated that far into Africa, long-tailed weasels are not the only species in South America.

Amazon weasels (Mustela africana - although I have no idea why) live throughout the Amazon jungle, and may be descended from long-tailed weasels that moved further south. They are about the size of a stoat, with dark fur over most of the body, pale underparts, and an unusual dark stripe running down the centre of their chest and belly. What little we know of them makes them seem slightly odd, since they appear to be active mainly during the day, and to travel in small groups, whereas most weasels are nocturnal and solitary. With so few reports of the animal, however, it's difficult to tell how much of this is really typical - it's presumably quite hard to find something the size of a weasel in the jungle at night.

The most recently discovered species of weasels are the Colombian weasels (Mustela felipei), first described only in 1978. Found close to high mountain rivers on both sides of the Colombian-Ecuadorian border, they have dark brown fur with yellowish underparts. Like most tropical weasels, their feet have hairless soles, but, unusually, the toes are also webbed, like those of a mink, although the animal is far smaller, scarcely larger than a common weasel. Presumably, this means that they are semi-aquatic, but we don't have enough observations to be sure. The only "true" weasel to be officially classified as a Vulnerable species, the animal is in danger almost before we've learned anything about it.

All of the species so far mentioned are placed within the genus Mustela, the largest of all weasel genera. There is some evidence that the three American species may actually be more closely related to American mink (Neovison vison) than they are to weasels on other continents, and, if so, they will need to change their scientific name. That's entirely plausible, since it would explain where the American mink came from, but it's interesting to note that there is one weasel that's already known to be odd enough to be placed in its own genus.

Patagonian weasels (Lyncodon patagonicus) have only 28 teeth, rather than the 34 found in every other species, and they also look rather different. They have grizzled grey fur over most of the body, with dark limbs and throat and head, but with a broad white stripe across their face and neck that makes them look rather like a stoat-sized honey badger. They inhabit the open steppes and scrubby woodlands of the Argentinian interior, and parts of Chile, where they apparently feed on burrowing rodents such as tuco-tucos, and the occasional bird. It would be interesting to know how such an odd weasel came to be living so far from any of its kin. Does it represent the last survivor of an earlier migration of weasels, pre-dating the American mink and their relatives, pushed out into the southern steppes, or is it just a really aberrant member of that group? For the time being, it remains a mystery.

[Pictures of long-tailed and Altai Mountain weasels from Wikimedia Commons. Picture of back-striped weasel from glopedia.com. Picture of Siberian weasel by "coniferconifer" released under the terms of the Creative Commons 2.0 license. Cladogram adapted from Koepfli et al. 2008.]